This invention relates generally to a transistor switching apparatus for use in the control of a D.C. motor, and particularly to: improvements in controlling a base current flowing through a base-emitter path of a power switch transistor with its emitter-collector path connected in series with the D.C. motor.
Although the transistor switching apparatus according to the present invention is applicable to a wide variety of apparatus for supplying pulse current or voltage from a D.C. voltage source to a load, the description will be made of a control apparatus for a D.C. motor which is utilized in an electrically driven vehicle such as a golf cart, only for purposes of discussion but not in a limiting sense.
As discussed in the prior U.S. Pat. No. 3,517,292, it is conventional to control the operation of a D.C. motor by varying a resistance connected in series with the motor and the power source to control the effective voltage delivered to the motor. In such control apparatus, however, due to the power losses consumed in the resistance, unwanted heat is generated in the resistance and the efficiency of the control apparatus inevitably lowers.
To avoid such power losses and inefficiency, improved control systems have been proposed in which a power switch transistor is connected in series with the D.C. motor and the power source. The power switch transistor is controlled to turn on and off alternatively by a pulse current applied thereto. The control of a rotational speed of the D.C. motor is effected by varying a duty ratio of the pulse current applied thereto so that the desired average voltage can be developed across the D.C. motor.
In order to reduce power losses in the power switch transistor, it is desirable for the transistor to operate in a fully saturated region of operation when an "on" pulse is applied, and in a fully cut off state when an "off" pulse is applied to the power transistor. As a result, the transistor must have sufficient base drive to operate in the switching mode for low power dissipation. Usually a base current of the power transistor is selected to be sufficient for rendering the transistor operative in the fully saturated region at the maximum load current, i.e. the maximum armature current of the D.C. motor. Accordingly the base current is relatively large in value, and, thus, power losses due to the base current in a base drive circuit becomes relatively large.
In order to avoid such power losses, prior U.S. Pat. No. 3,617,845 has proposed a variable controlled current supply system wherein the power transistor is supplied with a base drive current which varies in its value in accordance with a load current flowing through the D.C. motor.
It is noted, however, that both of the patents mentioned above are typical of the prior art in that no attention is paid to the fact that a D.C. amplification factor of the power transistor usually varies with increases in its collector current. As discussed later in detail, when the collector current is small, the D.C. amplification factor of the transistor is relatively large so that the base current required for the transistor to operate in the saturated state of conduction becomes relatively small. Conversely, when the collector current is large, the amplification factor of the transistor is relatively small so that the base current required for the transistor to operate in the saturated region becomes relatively large.
It should be further noted that the prior devices pay no attention to the fact that the voltage of power sources such as a storage batteries tend to reduce with time, and, therefore, the base drive current will become insufficient for the power transistor to be in the fully saturated condition of conduction over a long period of time.